Sections color changed with gradients

[ankurg22]

Please read and understand everything below Do not delete any text other than where you are instructed.

Students: If one of them is applicable to you. Please check it.

Check by changing each [ ] to [x] Please take note of the whitespace as it matters.

• [x] Read and understood (see CONTRIBUTING.md)
• [x] Included a Preview link and screenshot showning after and before the changes.
• [ ] Images are 240 x 240 [w x h].
• [x] Included a description of change below.
• [x] Squashed the commits.

Changes done in this Pull Request

• Changed colors of sections as specified in issue.
• https://ankurg22.github.io/gci17.fossasia.org/
• Closes #955

[ankurg22]

@mariobehling I made the changes. Need your feedback then will make changes accordingly. :+1:

[mariobehling]

I am also looking for the PR, that changed the top drop-down menu and made the color red on scroll over. As it does not fit into the design here, it should be reverted.